This invention relates to the production of copper. In one aspect, this invention relates to the pyrometallurgical production of copper while in another aspect, this invention relates to the conversion of copper matte to blister copper. In yet another aspect, this invention relates to reducing the sulfur content of blister copper while simultaneously reducing the copper oxide content of molten slag. In still another aspect, this invention relates to an apparatus for enhancing the natural mixing of phases that occurs at the interface of a blister copper layer and a slag layer within a copper converting furnace.
The pyrometallurgical production of copper is a series of multistep concentration, smelting, and refining procedures. Typically starting with an ore comprising one or more of a copper sulfide or copper-iron-sulfide mineral such as chalcocite, chalcopyrite and bornite, the ore is converted to a concentrate containing usually between 25 and 35 weight percent (wt %) copper. The concentrate is then converted with heat and oxygen first to a matte (typically containing between 35 and 75 wt % copper), and then to blister copper (typically containing at least 98 wt % copper). The blister copper is then refined, usually first pyrometallurgically and then electrolytically, to copper containing less than 20 parts per million (ppm) impurities (sulfur plus noncopper metals, but not including oxygen).
The conversion of copper concentrate to blister copper with heat and oxygen is known generally as smelting, and it comprises two basic steps. First, the concentrate is "smelted" to copper matte and second, the matte is converted to blister copper. Typically these steps are performed in separate furnaces, and these furnaces can vary in design. With respect to the first step, i.e. the smelting step, solid copper concentrates are introduced into a smelting furnace of any conventional design, preferably a flash smelting furnace, which is fired by the introduction of fuel and air and/or oxygen through a concentrate burner, and from which slag is tapped periodically and off-gases are routed to waste handling. In a flash smelting furnace, the copper concentrates are blown into the furnace through a burner together with the oxygen-enriched air. The copper concentrates are thus partially oxidized and melted due to the heat generated by the oxidation of the sulfur and iron values in the concentrates so that a liquid or molten bath of matte and slag is formed and collected in the basin (also known as the "settler") of the furnace. The matte contains copper sulfide and iron sulfide as its principal constituents, and it has a high specific gravity relative to the slag. The slag, on the other hand, is composed of gangue mineral, flux, iron oxides and the like, and it has a low specific gravity relative to, and thus floats on top of, the matte.
The molten copper matte and slag are separated in any conventional manner, typically by skimming the molten slag from the matte through tap holes in the furnace walls. The slag tapholes are located at an elevation on the furnace walls that allows slag withdrawal from the furnace without removal of molten matte. Tapholes for the molten matte are located at a lower elevation on the furnace walls that allows the withdrawal of molten matte without the withdrawal of slag. The molten copper matte is then either transferred directly or indirectly (e.g. by way of a holding furnace) to the converting furnace by any conventional means, e.g. launder or ladle, or its converted to solid form, e.g. granulated, for storage and later use as a feed to a converting furnace.
Converting furnaces are basically of two types, flash (also known as suspension) and bath, and the purpose of both furnaces is to oxidize, i.e. convert, the metal sulfides to metal oxides. Representative bath furnaces include those used by Noranda Inc. at its Horne, Canada facility, by Mitsubishi Materials Corporation at its Naoshima, Japan facility, and by Inco Limited at its Sudbury, Canada facility. Representative flash converting furnaces include that used by Kennecott Utah Copper Corporation at its Magna, Utah facility.
Regardless of its design, the converting furnace contains a bath of molten blister copper which was formed by the oxidation of copper matte that was fed earlier by one means or another to the furnace. The bath typically comprises blister copper of about fifty centimeters in depth upon which floats a layer of slag of about 30 centimeters in thickness. If the furnace is a rotary bath-type, then the molten metal and slag, separately of course, are poured from a mouth or spout on an intermittent basis. If the furnace is stationary, then outlets are provided for the removal of both the slag and blister copper. These outlets include tapholes located at varying elevations on one or more of the furnace walls and in a manner similar to that used with the smelting furnace, each is removed from the furnace independent of the other. Other furnace designs use one or more outlets that continuously overflow copper and slag (occasionally using, however, a weir to retain the slag).
The phase separation that occurs between the slag and blister copper is not complete and as such, the slag contains copper (usually in the form of cuprous oxide, i.e. Cu.sub.2 O, and copper metal, i.e. Cu.sup.0) and the blister copper contains various waste and unrecovered mineral values, e.g. sulfur (principally in the form of cuprous sulfide, i.e. Cu.sub.2 S), ferrosilicates, cuprous oxide, etc. The copper in the slag is lost metal value which is recovered by recycling the slag back to the smelting furnace. The waste and unrecovered mineral values in the blister copper are impurities which are eventually removed either in the anode furnace or through electrorefining.
The oxidation of copper sulfide at the interface of the slag and blister copper phases is known. However, the beneficial effect of this oxidation is minimized, particularly in stationary furnaces, by the relative quiescent state of the interface (because the activities of reacting sulfur and oxygen species must be high enough to produce sulfur dioxide at a pressure greater than that superimposed on the interface by the gas pressure in the furnace (about 1 atmosphere absolute) and the layer of slag above the interface (about 0.1 atmosphere absolute)). The oxidation will also be limited by the time in which the interface exists before the slag and blister copper are separated into different fractions.